This invention relates to digital signal switching systems and, more particularly, to such systems in which speech samples may be encoded into and decoded from pulse code modulation signals. In the pulse code modulation (PCM) arrangements to which the present invention is applicable, a sample of a speech signal is compared with a reference waveform. A counter, activated at the beginning of each sampling interval, is stopped when the reference waveform corresponds to the amplitude of the speech signal. The recorded count is then digitally transmitted to the remote terminal where the original speech signal may be reconstructed by employing the reference waveform and the received code in an inverse or decoding operation.
In J. F. O'Neill U.S. Pat. No. 3,860,761 issued Jan. 14, 1975, a digital progressive switching system is shown in which a single wire carries digitally-encoded voice signals and network control signals during predetermined time slot intervals. In the copending application of J. F. O'Neill Ser. No. 687,622, filed May 19, 1976 of even date herewith, an improved digital transmission system is shown in which the transmission delay and signal attenuation introduced in sending an encoded speech sample to a remote station of the telephone switching system may be compensated for by employing a delayed reference waveform having a different amplitude in the decoding operation than is employed in encoding the speech sample. Both the encoding and decoding waveforms that are employed are each advantageously nonlinear or companded so as to provide an acceptable signal-to-noise ratio over the entire dynamic range of the signal.
The use of a nonlinear or companded reference waveform, however, tends to introduce an undesired amount of nonlinear distortion. Because the reference waveform is steepest at both ends of the voltage comparison range (as it must be to compensate for the effects of quantizing noise), any finite delay in the operation of the physical circuit components occurring at these times will directly introduce nonlinear distortion. Such nonlinear distortion will be introduced, for example, by the finite and unavoidable delay in shutting off the counter when the reference waveform is recognized as being equal to the amplitude of the speech sample. Similarly, nonlinear distortion will be introduced by the circuit "reaction time" involved in disconnecting the decoding capacitor from the reference waveform when the received digital code is recognized.
In the aforementioned O'Neill U.S. Pat. No. 3,860,761 system, it was assumed that the digital encoding and decoding operations at a station set occurred during certain conveniently-assigned intervals of time. It would be advantageous, however, to permit the station set or line circuit to perform certain of these operations on an overlap basis. For example, it would be efficient to permit such a circuit to perform the coding operation for a to-be transmitted sample at the same time that it is receiving a signal over the link. During another interval of time, it would be useful to permit that same station to decode the signal it had received over the link at the same time that it transmits over the link the signal which it had previously encoded. To accomplish these overlap operations, the station at the distant end of the link must be performing the complementary pairs of simultaneous operations. It would be desirable to provide a circuit that permitted the aforementioned overlap operations to be performed so that whatever steps need be taken to correct for the previously-mentioned and unavoidable nonlinear distortion effects could be accomplished without interfering with such overlap operations.